Growth and mortality of red sea urchins Strongylocentrotus franciscanus across a latitudinal gradient

Abstract

Growth and survival of the red sea urchin Strongylocentrotus franciscan us were studied at 18 sites from southern California to Alaska, USA. Growth was determined using tetracycline tagging and was modeled using the Tanaka growth equation. Survival rates were estimated using size-frequency distributions and growth parameters. Using log-linear analysis, it was determined that growth transitions differed among sites (p << 0.001) but there was no north-south difference (p > 0.80). Parameters for the Tanaka growth function were estimated for all data combined (N = 2714). Residuals for sites showed no latitudinal trend and so results were consistent with the log-linear analysis. Relative jaw (demi-pyramid) size, measured as the allometric exponent β in jaw length as a function of test diameter, has been shown to be responsive to available food. For red sea urchins, β was negatively correlated with growth but there was no correlation of relative jaw size with latitude, which suggests that latitudinal differences in food availability do not exist. In contrast with annual growth rates, annual survival rates were correlated with latitude and were higher in the north. Mean annual survival probability was 0.93 yr-1 from northern California to Alaska and 0.77 yr-1 in southern California. Likely causes for changes in survival rate with latitude are disease and temperature-related stress. This paper provides the basis for development of hypotheses for size and survival differences between northern and southern populations of red sea urchins and, potentially, for other marine species with planktonic larvae.